1. Field of the Invention
2. State of the Art
The present invention relates to a digital video signal monitoring apparatus.
In order to ensure that a television transmission/reception system is operating satisfactorily, various apparatus have been devised which enable an engineer to monitor a received video signal.
Hitherto, due to limitations in the speed and complexity of available electronic hardware, the majority of transmitted video signal have comprised analogue signals which are typically monitored sing an oscilloscope or vectorscope. More recently, however, a number of systems have been disclosed which allow analogue video signal waveforms to be displayed superimposed over, or xe2x80x98burntxe2x80x99 into a decoded video image (i.e. the video image is blacked out at the locations of the video signal waveform displays). This arrangement is clearly advantageous, particularly in that the engineer is able to view both the video signal waveform and the video image simultaneously.
Digital broadcast video systems are now commonplace in the television industry with suitable digital technology having been available for some time.
A typical digital video signal comprises a plurality of digital signals, such as luminance, two color difference and an audio signal, multiplexed together.
Each of the signals making up the digital video signal comprises a plurality of successive digital sample values, so that when monitoring those signals it has been necessary either to display their waveforms as a series of discrete points corresponding to their respective sample values, or to low-pass filter the digital signal waveforms so that the resulting analogue signal waveforms may displayed.
Whilst it is often more desirable to display a continuous signal waveform rather than its discrete digital equivalent, the process of converting a signal between the digital and analogue domains leads to inaccuracies in its displayed waveform.
Also, where it necessary to re-digitise an analogue signal, for example to enable the signal waveform to be displayed superimposed over, or xe2x80x98burntxe2x80x99 into a decoded video image, then the analogue-to-digital/digital-to-analogue conversion processes will tend to introduce unnecessary noise into the signal which may corrupt the decoded video image.
I have now devised an arrangement which allows digital video signal waveforms to be displayed without the necessity for conversion to an equivalent analogue signal, whilst providing the improved appearance of a continuous analogue waveform display.
In accordance with the present invention, there is provided a digital video signal monitoring apparatus comprising means for receiving a digital video signal comprising a plurality of digital signals which have been multiplexed together, each signal comprising a plurality of successive digital sample values, means for extracting from said digital video signal one or more of said plurality of digital signals, means for interpolating additional digital values intermediate the successive samples of the or each extracted digital signal such that its sample rate is increased, and means for displaying the waveform the or each extracted digital signal as a series of discrete paints corresponding to its respective digital sample values and interpolated digital values.
Thus, the increased resolution of the or each displayed signal waveform gives it a more continuous appearance. However, none of the original signal information is lost or distorted, and so the decoded video image may be displayed without loss of quality.
Preferably, said plurality of digital signals comprises a luminance signal and/or a pair of color difference signals and/or an audio signal.
Preferably said monitoring apparatus comprises means for selecting which of said plurality of digital signals is to have its waveform displayed.
Preferably the waveform of the or each extracted digital signal may be displayed as a linear or vector display. Preferably the monitoring apparatus comprises means for combining the displayed signal waveform with a graticule or other graphical measurement or reference means. Preferably digitally generated horizontal and vertical cursors are provided to allow amplitude and/or timing measurements to be made of a displayed waveform. Preferably the results of said amplitude and/or time measurements are numerically displayed by said display means.
The apparatus may also comprise means for decoding the luminance and color difference signals to obtain corresponding RGB values which may be displayed in the form of a gamut display, and means to identify therefrom the presence of any illegal colors.
Preferably the interpolating means comprises an oversampling digital filter which preferably comprises a transversal FIR (finite impulse response) filter whose coefficients are derived from a sin(x)/x low-pass impulse response curve. Preferably the transversal filter comprises five filter stages, the degree of asymmetry of whose respective coefficients determines the magnitude of each intermediate digital value.
The intermediate digital values may be interpolated by varying the coefficients of the oversampling digital filter, on a line by line basis, over a complete video field, preferably such that between the first and last lines of the video field, the sin(x)/x curve from which the coefficients are derived is phase shifted in discrete intervals, between xe2x88x92180 degrees and +180 degrees. A complete signal waveform may then be obtained by superimposing the filter outputs for each line.
Alternatively, the intermediate digital values may be interpolated by recirculating a single line of the video field through a buffer whilst varying the coefficients of the oversampling filter by phase shifting the sin(x)/x curve from which the coefficients are derived.
Preferably the or each digital signal waveform is displayed superimposed over, or xe2x80x98burntxe2x80x99 into, a decoded video image.
Preferably said display means comprises a cathode ray tube (CRT), which may be television or computer type monitor e.g. VGA, SVGA. However said display means may instead comprise a non-CRT devise such as a liquid crystal display (LCD), plasma display or projection system, etc.
The or each digital signal waveform may be displayed directly or may be re-transmitted for display at a remote location.
The apparatus lay further comprise various error detection means for identifying and preferably logging a variety of errors in the received digital video signal, such as clock jitter, missing or invalid synchronisation codes, cyclic redundancy errors, etc. Error messages may be numerically displayed by said display means.
An embodiment of the present invention will now be described by way of example only and with reference to the accompanying with drawings, in which: